The present invention relates to electric lawn mowers and in particular to a cordless electric lawn mower having an electronic energy management control system for maximizing the efficiency of the mower.
Due to concerns regarding urban air pollution, as well as other factors, electric lawn mowers are gaining in popularity. Moreover, due to the inconveniences and operating limitations of corded electric mowers, battery operated or cordless electric mowers are preferred. However, the principal disadvantage of cordless electric mowers is the run time of the mower on a single battery charge. For obvious reasons, it is highly desirable for a cordless electric mower to possess the capacity to complete the mowing of a typical size lawn on a single charge. A run time of approximately one hour is generally required for this purpose. Unfortunately, and often times contrary to claimed performance, most cordless electric mowers have a real world run time of approximately one-half hour which is usually insufficient to complete the desired task.
Although use of a larger battery would certainly overcome this problem, the cost and weight penalties associated with larger batteries preclude their use on non-commercial mowers. Consequently, the size and weight of the battery that can be used is limited. Present cordless electric mowers typically employ a 24-volt lead-acid battery with a rated capacity of approximately 20 amp-hours. However, not all of this energy is available for use. For example, with lead-acid batteries, 80% efficiency is as high as can realistically be achieved with any appreciable load on the motor. Consequently, give a 24-volt lead-acid battery rated at 20 amp-hours, the maximum available energy from the battery is practically limited to approximately 17 amp-hours. Moreover, when the level of current draw on the battery is increased, the efficiency of the battery decreases even further to 50% or less. This phenomenon thus also greatly affects the total available run time of the mower under normal operating conditions.
Therefore, in order to substantially improve the operating time of a cordless electric mower, given the available battery energy, it is necessary to consider the total efficiency of the mower, including not only the efficiency of the motor, but also the efficiency of the battery, as well as the efficiency of the cutting blade. Conventional techniques for improving mower efficiency have generally focused on the motor by attempting, for example, to minimize friction losses with more expensive bearings. However, the efficiencies gained with these approaches are limited and therefore of questionable value given their associated costs. Generally overlooked in these efforts to improve overall mower operating efficiency is the impact of blade speed.
In particular, most rotary powered lawn mowers, whether gas or electric powered, typically rotate the cutting blade at a speed of between 3800-4000 rpm. This speed provides very good cutting results, including in mulching and bagging applications. However, at a no-load blade speed of 3800 rpm, the blade speed is the greatest contributing factor to the overall efficiency of the mower. This is because the total watts consumed by the mower is a function of the cube of the blade speed. Accordingly, even though the functional results achieved by operating the cutting blade at these speeds are very good, the level of energy required to do so is also very high.
On the other hand, good cutting results can still be achieved at blade speeds of approximately 3000 rpm. Moreover, substantially less energy is required to operate the mower at such cutting blade speeds. For example, under a no-load condition, it requires approximately 543 watts of power to rotate the cutting blade at 3800 rpm, but only 218 watts of power to rotate the cutting blade at 3000 rpm. Accordingly, significant improvements in overall efficiency can be achieved by controlling the rotational speed of the motor so as to limit the speed of the cutting blade.